As silicone compositions curable with ultraviolet radiation and electron beams, organopolysiloxanes containing (meth)acryloxy groups in the molecule are known, and compositions comprising the same in admixture with sensitizers are also known from JP-B 53-36515. However, since the organopolysiloxanes in these compositions have a high content of (meth)acryloxy groups, the cured coatings are so hard that they do not smoothly follow the underlying substrates and are liable to cracking.
JP-A 61-232 and 56-86922 disclose a method for preparing an acrylic functional, UV-curable polysiloxane by adding an acrylic acid to an epoxy group-bearing organopolysiloxane. Since some acrylic acid is left unreacted, the cured coating has poor moisture resistance and electrical properties due to the influence of carboxyl groups left therein. If the amount of acrylic acid blended is reduced, some epoxy groups are left instead. As a result, the surface layer of cured coating is exacerbated in weather resistance and acid resistance by epoxy groups.
As a method of preventing acrylic acid from being left in radiation-curable coating compositions, JP-A 5-309664 discloses to introduce (meth)acryl groups into an organopolysiloxane by effecting dealcoholysis between alkoxysilyl groups and a (meth)acryl group-bearing silanol compound in the presence of a tin catalyst. The (meth)acryl group-bearing silanol compound, however, is difficult to synthesize and expensive from the commercial aspect. The residual tin catalyst is undesirable from the safety aspect.
Other methods known for the preparation of (meth)acryl functional organopolysiloxanes of this type include dehydrochlorination reaction between .tbd.Si--Cl groups and a hydroxyl group-bearing acrylate compound, dehydrochlorination reaction between a carbinol group-bearing siloxane and acrylic chloride, and hydrosilylation reaction between .tbd.Si--H groups and allyl (meth)acrylate. These methods suffer from problems such as difficult removal of by-products or hydrochloride salts, and complex reactions including dehydrogenation condensation by side reaction during hydrosilylation and multi-stage reaction. These methods are not regarded as commercially simple and inexpensive.
For improving the compatibility with other acrylic compounds and the transparency of resins, it is known to introduce phenyl groups into silicone compounds. This is conventionally carried out by equilibration using octaphenylcyclotetrasiloxane, with the drawback that much cyclic diphenylsiloxane is left in the synthesized polysiloxane.